1) Field of the Invention
The present invention relates to a device (hereinafter “width detecting device”) for detecting a width of a sheet-like medium for use, for example, in a paper feeder of an image formation apparatus such as a copying machine, a printer, a facsimile, or the like.
2) Description of the Related Art
A width detecting device for detecting a width of a sheet-like medium (i.e., a paper) has been disclosed in, for example, Japanese Patent Application Laid Open No. 5-43058. When it is assumed that the paper has a paper length direction and a paper width direction, then in this width detecting device, two limiting members are disposed parallel to the paper width direction and facing with each other on a base for placing the paper. These limiting members are arranged so as to move towards each other or away from each other, along the paper width direction via an interlocking unit. The limiting members are moved so that they make a contact with the respective edges of the paper, and a detecting unit detects the position of the limiting members to thereby detect the width of the paper.
The detecting unit includes a plurality of resistors having different resistances positioned at predetermined positions in the base along the paper width direction, and one contact element is positioned in each of the limiting plates. When the limiting plates are moved, the contact elements in the limiting plates make an electrical contact with the resistors in the base, and an electrical path is created. The detecting unit further includes a voltmeter that measures the voltage of this electrical path. Based on the voltage, it is possible to decide with which resistors the contact elements have made the electrical contact and thereby it is possible to decide the width of the paper because the positions of the resistors are known.
In this configuration there is a problem that the contact (hereinafter “contact pressure”) between the resistor and the contact element varies depending upon variation in the size of the parts, conditions at the time of assembly, and environmental conditions.
If the contact pressure is strong, the limiting plates may move because of the contact pressure, or it may become difficult to move the limiting plates because of the contact pressure. If the contact pressure is weak, an error may be generated in the detection of the width of the paper. If it is incorrectly detected, for example, that the paper is wider, an unnecessary portion is developed so as to increase the load on a cleaning device or excessive consumption of a toner can be generated. Moreover, since a relatively large resistor area is needed, the miniaturization of the width detecting device is not satisfied.
As a sensor for detecting an amount of movement (hereinafter “moving amount”) of a subject, a rotation angle detection type sensor that converts the moving amount into an angle and generates a signal corresponding to the angle is known. Some prior arts have used the rotation angle detection type sensor in combination with the limiting plates described above to detect the width of the paper. In that case, the rotation angle detection type sensor converts the amount of movement of the limiting plates into angles and determines the width of the paper.
The rotation angle detection type sensor can accurately detect the width of the paper when the angle is less than 360°, but can not properly detect the width when the angle is more than 360°. Because, since the pattern of the signal output by the rotation angle detection type sensor when the angle is less than 360° is the same as when it is greater than 360°, it is impossible to distinguish whether the angle is less than 360° or greater than 360°.
A width detecting device of manual feeding paper base is disclosed in, for example, Japanese Patent Application Laid Open No. 11-130271. This width detecting device has a rotation type variable resistor whose resistance varies according to a rotation angle of a shaft section. This width detecting device detects the paper width based on the resistance output by the rotation type variable resistor.
Generally, a paper of size A3 or smaller is used in the image formation device. It is possible to detect the width of the A3 size paper with the rotation angle detection type sensor or the rotation type variable resistor because the angle corresponding to the A3 size paper is less than 360°.
However, recently, there is an increasing demand to increase the paper size that can be handled in the image formation apparatus. If the size of the paper is increased, then the angle corresponding to that paper is greater that 360°, and the conventional rotation angle detection type sensor or the rotation type variable resistor do not give proper results. To get proper results, a gear used in the rotation angle detection type sensor should be made larger. If the gear is made larger, then extra space is required to accommodate the gear. Moreover, the number of switches used in the rotation angle detection type sensor increase. This leads to cost increase.